Grease in elastohydrodynamic lubrication
Abstract: The rheology of soap thickened greases has been investigated in a cone and plate rheometer and a model that includes base oil viscosity and temperature has been suggested and fitted to the results. With the use of a bouncing ball apparatus, the pressure coefficient of the limiting shear stress of the greases was determined. It is related to friction in EHL and the results verified the friction reducing properties of the soap thickeners of the grease. Film thickness in EHL of a grease lubricated point contact was measured in a ball and disc apparatus. The initial film thickness was thicker than the film thickness given by the base oil. After some seconds of running the film thickness dropped far below the base oil film thickness, because of insufficient replenishment. Replenishment was studied by looking at the distribution and flow of lubricant around the contact in the ball and disc apparatus. Primary reservoirs and primary bands close to the track of the contact, on the ball and the disc, contained small amounts of grease that constituted a potential supply of lubricant to the inlet of the contact. The majority of the grease was pushed away from the contact initially and seemed no to participate in the lubrication. Soap thickener formations were observed entering the contact in the ball and disc apparatus and cause a local increase in the film thickness that disturbed the normally horseshoe-shaped film thickness map. For lithium greases, the amount and size of the film disturbances were found to be connected with the manufacturing process of the grease and the soap thickener content. A method to evaluate the pressure fluctuations induced by the film thickness disturbances was proposed. The method was verified by comparing film thickness measurements from an oil lubricated contact and the pressure profile calculated from that, with the results from a fully numerical solution. Good agreement was found. The method was applied to a grease lubricated contact and showed significant variations in the pressure, caused by the soap thickener formations.
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